An organic electroluminescent device has a multilayer structure comprising (i) a luminescent layer comprising a light emitting compound, (ii) a hole transport layer and an electron transport layer, which sandwich the luminescence layer, and (iii) an anode and a cathode, which sandwich the hole transport layer, the luminescent layer and the electron transport layer. The organic electroluminescent device utilizes light emission (fluorescence or phosphorescence) occurring at deactivation of an exciton formed by the binding of electron with hole, which are injected in the luminescence layer.
In recent years, a wide spread attention is attracted to an organic electroluminescent device for next-generation flat panel displays. This is because, first, an electroluminescent device can be made into a thin film and be rendered light in weight; secondly, power consumption is small due to the spontaneous light emission; and thirdly, the device structure is simple and thus the production cost is low. Various methods can be adopted for the production thereof, which include, for example, vacuum deposition, spin coating, ink-jet printing, off-set printing and thermal transfer printing.
Now various mobile devices such as cell phones, mobile music devices, and personal digital assistant (PDA) are widely used. However, if mobile devices can be larger in size or more precise, organic electroluminescent devices are expected to be used in, for example, flat panel displays, lighting systems with a surface-light-emitting source, flexible paper-like displays, wearable displays and transparent see-through displays. Its use is expected to be rapidly spread.
However, an organic electroluminescent device still has many technical problems to be solved. Especially its driving voltage is high and the efficiency is low, and thus, its power consumption is large.
The above-mentioned technical problems arise due to the property of the material of organic electroluminescent device, especially the property of electron transport material. Many materials including triarylamine derivatives have been proposed as a hole transport material, but, only several reports are found as to the electron transport material. Tris(8-quinolinolato)aluminum (III) (Alq) is already put in practical use as an electron transport material, but, its property is poor as compared with a hole transport material such as, for example, N,N′-bis(1-naphthyl)-N,N′-diphenyl-4,4′-biphenyl (NPD), and an organic electroluminescent material comprising the electron transport material has also poor property.
As other electron transport materials, there can be mentioned oxadiazole derivatives (patent document 1), quinoxaline derivatives (patent document 2), triazole derivatives (patent document 3), silacyclopentadiene derivatives (patent document 4), quinoline derivatives (patent document 5), benzimidazole derivatives (patent document 6) and benzothiazole derivatives (non-patent document 1). However, organic electroluminescent devices comprising these electron transport materials still have problems in that their driving voltage is high, the film are readily crystallized, and their service life is short.
Recently, 1,3,5-triazine compounds have been proposed as other electron trhansport materials (patent documents 7, 8, 9, 10 and 11).
Patent document 1: JP-A H6-136359
Patent document 2: JP-A H6-207169
Patent document 3: WO95/25097
Patent document 4: JP-A 2005-104986
Patent document 5: JP-A 2006-199677
Patent document 6: WO2004/080975
Patent document 7: JP-A 2003-045662
Patent document 8: JP-A 2003-282270
Patent document 9: JP-A 2004-022334
Patent document 10: U.S. Pat. No. 6,225,467
Patent document 11: U.S. Pat. No. 6,352,791
Non-patent document 1: Applied Physics Letters, vol. 89, 063504, 2006